Pathogenic microbes and pests are responsible for substantial economic losses in crop production worldwide. Current control practices against them each have severe drawbacks. In principle, breeding new varieties of crops, which are inherently more resistant to the pathogens, can prevent crop losses. In practice, however, each new variety is ultimately doomed to fail since pathogens slowly evolve a resistance. Application of synthetic non-natural chemicals pose a significant risk for ecology. Only very recently related molecules existing in nature have been introduced. However, such natural chemicals are expensive, demand special spraying tools and are labor-intensive.
During the last 2 decades, new approaches involving transgenic plants with certain alien genes have been developed to generate resistance to viral pathogens. Such plants involve, as a rule, expression of certain viral genes (for example, coat protein). Unfortunately, the acquired resistance is only effective against the specific viral strain that the plant is “vaccinated” against. For example, resistance to potato virus Y (PVY) was ineffective against other viral strains which differed by as little as 22% at the nucleotide level. Hence, this kind of resistance has limited practical applications because different pathogens dominate when the climate conditions and other factors change. Farmers expect that their investment in the costly seed materials should be profitable each year, not only during some years. Nevertheless, because of the lack of more universal solutions, such limited resistance has been engineered against different viruses in a wide range of crop species. Also the situation has to be seen from the increasingly important perspective of public opinion and fear regarding the use of transgenic plants: benefits must be positive enough to outweigh public concerns.
Systemic acquired resistance (SAR) is a resistance reaction first reported by Chester (1933). SAR is a common plant defense reaction in which a plant systemically produces various defense molecules such as lignin, phytoalexins and PR-proteins to prevent the spread of pathogens (reviewed by Sticher et al. 1997, Ann. Rev. Phytopathol. 35, 235-270). SAR can be induced within a few hours by many pathogenic microbes and the resistance then lasts for several weeks. SAR is a salicylic acid-dependent resistance reaction, but the primary role of salicylic acid in SAR is still unclear (Ryal et al. 1996, Plant Cell 8, pp. 1809-1819). Spread of the pathogen is confined to a small number of cells in plants with an established SAR; therefore the pathogens cannot, in practice, harm the plant.
Whereas there exists an enormous necessity for quick development of novel plant species with higher resistance against different diseases and parasites, and with a higher ecological safety, no such solution has been clearly offered. The present invention offers completely novel concepts for increasing the plant resistance based on our surprising finding that a certain protein from bacterial strains, the proteins termed as MF3, can trigger in plants a wide systemic resistance against including viruses, bacteria, fungi, insect pests, such as the Colorado potato beetle, and nematodes. The broad protective potential of MF3 is demonstrated by the experimental results justifying the claim that the protein can generate a full spectrum of plant resistance to pathogens and pests. While the effects of the discovered protein in plants resemble those of SAR, no mechanistic connection can be drawn for their relationship.
We discovered previously another protein with a distinctly different molecular structure than MF3; this previous protein originated from a Bacillus thuringiensis strain (MF2, Djavakhia V., et al. U.S. Pat. No. 6,528,480). Transgenic tobacco plants expressing MF2 possessed increased resistance to viruses and fungi (Tobacco Mosaic Virus and Alternaria longipes). In the present invention we found a totally different microbial protein with improved activity and significantly wider applications. Therefore the present invention provides a significant improvement over our previous invention by showing that a novel molecule can induce multiple resistance to plants involving microbes, as well as insects and nematodes that, in particular, are known as serious plant parasites. We also show that MF3 can be used as the resistance inducer in various transgenic plants without any loss of crop productivity.